This application is based on and claims priority to Japanese Application No. 2001-119380, the entire disclosure of which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to a power transmission device of an all terrain vehicle (ATV). More particularly, the present invention relates to a power transmission device of an all terrain vehicle comprising a V-belt continuously variable transmission (CVT) and a gear-type transmission which are arranged in this order from upstream in a power transmission path from an engine to wheels, and an all terrain vehicle comprising the power transmission device.
2. Description of the Related Art
In the all terrain vehicle comprising the V-belt continuously variable transmission and the gear-type transmission, the V-belt continuously variable transmission is adapted to perform automatic speed change according to an engine speed and increase/decrease in load of wheels, and some gear-type transmissions are adapted to perform switching between forward and reverse, between forward high-speed and forward low-speed, and the like, by operation of a hand-operated shift lever.
In this all terrain vehicle, a braking effect of the engine is commonly produced by a friction force generated between a V belt and sheaves in the V-belt continuously variable transmission. Specifically, torque can be transmitted from wheel side to engine side while engine braking.
The all terrain vehicle travels over uneven roads, mountains, steep slopes, rough terrains, shores, and the like, and therefore, the engine brake is applied more frequently than those of general vehicles.
However, when the braking effect of the engine is produced by the friction force between the V-belt and the sheave of the V-belt continuously variable transmission as described above, because the coefficient of friction changes with wear of the belt, and changes in conditions of temperature, moisture, and the like, the braking effect of the engine varies, and excessive engine braking effect, for example, may result, which does not produce a comfortable driving feel.
On the other hand, where as a countermeasure to prevent excessive engine braking effect, an average braking effect of the engine is set relatively lower by lowering a belt pressing force of a movable sheave during engine braking, the engine braking effect is somewhat insufficient, which does not produce a comfortable driving feel.
As prior art directed to controlling the engine brake, a power transmission device is disclosed in Publication of Unexamined Patent Application No. Sho. 63-186043. In this power transmission device, as shown in FIG. 1, power of the engine E is transmitted from a transmission T to right and left front wheels 201 through a front-wheel differential unit 200. The power is further transmitted from bevel gears 202, 203 to a viscous clutch 205 through a propeller shaft 204, and to rear wheels 212 through a clutch device 210 for controlling the engine brake and a rear-wheel differential unit 211.
As shown in FIG. 2, in the clutch device 210 for the engine brake, a one-way clutch 214 and a multi-plate-friction-type limit clutch 213 are provided in parallel between two rotational shafts 206, 207 provided in series on the side of the engine E and on the side of wheels 201, 212, respectively. It should be appreciated that this conventional all terrain vehicle does not employ the technique for producing the braking effect by the friction force between the belt and the sheave, because of absence the V-belt continuously variable transmission.
The one-way clutch 214 and the limit clutch 213 are located downstream of the transmission T, the front-wheel differential unit 200, the propeller shaft 204, and the viscous clutch 205 in a power transmission path from the engine E. For this reason, the transmitted torque becomes larger as it goes from the engine E toward the downstream in the power transmission path, and hence, the limit clutch 213 of large friction capacity and the one-way clutch 214 of large transmission capacity are needed. Consequently, the clutch device for controlling the engine brake becomes large-sized and its cost is increased.
The present invention addresses the above-described conditions, and an object of the present invention is generally to improve the comfort and feel of driving for an all terrain vehicle comprising a V-belt continuously variable transmission and a gear-type transmission. To be specific, the object is as follows.
(1) Transmitted torque from the wheel generated while engine braking can be controlled with higher precision without loss of drive torque transmitted from the engine.
(2) Even when there is some fluctuation in the torque transmitted to the V-belt continuously variable transmission while the engine braking, the production of excessive engine braking effect is prevented, thereby maintaining average braking effect at high level and stabilizing the engine braking effect.
(3) A torque limiter and a one-way clutch for controlling the engine brake are small-sized to be accommodated compactly. In addition, forced lubrication for the torque limiter and the one-way clutch is conducted in a simplified manner.
To achieve the above-described object, according to the present invention, there is provided an all terrain vehicle comprising a V-belt continuously variable transmission and a gear-type transmission placed in this order from upstream in a power transmission path from an engine to wheels, in which a one-way clutch for transmitting only drive torque from the engine to the wheel and a torque limiter for transmitting torque with a predetermined value or less are arranged in parallel between a transmission shaft in the gear-type transmission and a transmission gear fitted to the transmission shaft.
By arranging the one-way clutch and the torque limiter in parallel in this manner, the transmitted torque from the wheel generated while the engine braking can be controlled with higher precision without loss of the drive torque transmitted from the engine to the wheel.
Even when there is some fluctuation in the torque transmitted to the V-belt continuously variable transmission during engine braking, the torque limiter functions to prevent production of the excessive engine braking effect. Thereby, the average engine braking effect can be maintained at high level, and the braking effect can be stabilized.
Further, since the torque limiter and the one-way clutch are placed in the gear-type transmission, the transmitted torque is smaller in the present invention in contrast to the case where they are provided downstream in the power transmission path, e.g., in a differential unit of wheels or on a propeller shaft. Therefore, the torque limiter and the one-way clutch can be small-sized and can be easily placed without ensuring a special space. Besides, forced lubrication oil in the gear-type transmission case can be directly utilized as lubricating oil for the torque limiter, which eliminates a need for special pipings.
The one-way clutch and the torque limiter are provided to a transmission intermediate shaft placed between a transmission input shaft and a transmission output shaft, among the transmission shafts in the gear-type transmission. In accordance with this, the torque limiter and the one-way clutch can be easily placed by making use of an empty space around the intermediate shaft in contrast to the case where these components are provided to a shaft commonly provided with a gear shift mechanism or the like, e.g., the transmission input shaft. In addition, since the torque necessary for the transmission intermediate shaft is smaller than that of the transmission output shaft, the torque limiter and the one-way clutch can be small-sized.
The torque limiter is comprised of an inner peripheral tapered face of the transmission gear, a cone member having an outer peripheral tapered face being pressed against the inner peripheral tapered face, and biasing means for causing these tapered faces to be pressed against each other by a given force. In this constitution, the torque limiter can be placed compactly between the transmission gear and the transmission shaft. Preferably, the biasing means may be a coil spring to make the torque limiter simple. The biasing means also may be those utilizing hydraulic pressure.
Two cone members of the torque limiter may be provided on both sides of the one-way clutch in the axial direction. This makes it possible for the cone members to function as bearings to be provided on both sides of the torque limiter. Therefore, the number of parts can be reduced and a more compact constitution is attained.
The torque limiter may be a multi-plate friction-type torque limiter. Thereby, the torque capacity can be increased while keeping radial dimension compact.
The above and further objects and features of the invention will more fully be apparent from the following detailed description with accompanying drawings.